A rationale for cone beam CT with extended longitudinal field-of-view in image guided adaptive radiotherapy

Abstract Purpose To investigate the efficacy of using cone beam CT with extended longitudinal field-of-view (CBCT eLFOV ) for image guided adaptive radiotherapy (IGART). Methods The protocol acquires two CBCT scans with a linear translation of treatment couch in the patient plane, allowing a 1 cm penumbral overlap (i.e. cone beam abutment) and fused as a single DICOM set (CBCT eLFOV ) using a custom-developed software script (coded in MatLab®) for extended localization. Systemic validation was performed to evaluate the geometric and Hounsfield Units accuracy at the overlapping regions of the CBCT eLFOV using a Catphan®-504 phantom. Two case studies were used to illustrate the CBCT eLFOV -based IGART workflow in terms of dosimetric and clinical perspectives. Segmentation accuracy/association between repeat CT (re-CT) and CBCT eLFOV was evaluated. Moreover, the efficacy of the CBCT eLFOV image data in deformable registration was also described. Results Slice geometry, spatial resolution, line profiles and HU accuracy in the overlapping regions of the CBCT eLFOV yielded identical results when compared with reference CBCT. In patient studies, the dice-similarity-coefficient evaluation showed a good association (>0.9) between re-CT and CBCT eLFOV . Dosimetric analysis of the CBCT eLFOV -based adaptive re-plans showed excellent agreement with re-CT based re-plans. Moreover, a similar and consistent pattern of results was also observed using deformed image data (initial planning CT deformed to CBCT eLFOV ) with extended longitudinal projection and the same frame-of-reference as that of the CBCT eLFOV . Conclusion Utilization of CBCT eLFOV proves to be clinically appropriate and enables accurate prediction of geometric and dosimetric consequences within the planned course of treatment. The ability to compute CBCT eLFOV -based treatment plans equivalent to re-CT promises a potential improvement in IGART practice.